Influence of Enhanced Abyssal Diapycnal Mixing on Stratification and the Ocean Overturning Circulation

Abstract

The meridional overturning circulation (MOC) is composed of interconnected overturning cells that transport cold dense abyssal waters formed at high latitudes back to the surface. Turbulent diapycnal mixing plays a primary role in setting the rate and patterns of the various overturning cells that constitute the MOC. The focus of the analyses in this paper will be on the influence of sharp vertical variations in mixing on the MOC and ocean stratification. Mixing is enhanced close to the ocean bottom topography where internal waves generated by the interaction of tides and geostrophic motions with topography break. It is shown that the sharp vertical variations in mixing lead to the formation of three layers with different dynamical balances governing meridional flow. Specifically, an abyssal bottom boundary layer forms above the ocean floor where mixing is largest and hosts the northward transport of the heaviest waters from the southern channel to the closed basins. A deep layer forms above the bottom layer in which the upwelled waters return south. A third adiabatic layer lies above the other two. While the adiabatic layer has been studied in detail in recent years, the deep and bottom layers are less appreciated. It is shown that the bottom layer, which is not resolved or allowed for in most idealized models, must be present to satisfy the no flux boundary condition at the ocean floor and that its thickness is set by the vertical profile of mixing. The deep layer spans a considerable depth range of the ocean within which the stratification scale is set by mixing, in line with the classic view of Munk in 1966.

The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

“…Previous geothermal heating studies that were run for millennial time scales were not fully coupled (e.g., Adcroft et al 2001;Emile-Geay and Madec 2009), and thus lacked atmospheric feedbacks on the ocean that arise when geothermal heating impacts the ocean surface (e.g., Adcroft et al 2001;Mashayek et al 2013;Piecuch et al 2015). Here we show that geothermal heating-induced anomalies do extend toward the surface (thus impacting surface water mass transformation), particularly in polar regions where approximated atmospheric forcing errors can be substantial (e.g., Nygard et al 2016;Hobbs et al 2016).…”

“…Geothermal heating has a nonnegligible influence on the large-scale abyssal circulation by weakening abyssal stratification and increasing the circulation of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) on the order of 10%-30% (e.g., Adcroft et al 2001;Hofmann and Morales Maqueda 2009;Emile-Geay and Madec 2009;Mashayek et al 2013;de Lavergne et al 2016). On a regional scale, geothermal heating can change ocean bottom temperatures by an order of magnitude more than error estimates associated with decadal abyssal temperature trends (e.g., Emile-Geay and Madec 2009;Purkey and Johnson 2010;Kouketsu et al 2011;Wunsch and Heimbach 2014) and increase thermosteric sea level by 0.1-1 mm yr 21 (Piecuch et al 2015).…”

“…Few studies have focused on the Southern Ocean, where geothermal heating increases poleward heat transport by ;10% (Emile-Geay and Madec 2009) and further steepens isopycnals associated with Southern Ocean upwelling (Mashayek et al 2013). Several model studies also lack realistic topography (essential for representation of stratification along the ocean floor and midocean ridges outputting high geothermal heat), and none to date uses a fully coupled model configuration where atmospheric-oceanic feedbacks are included.…”

“…Here we assess two geothermal heat flux datasets, namely that of Hamza et al (2008) and Davies and Davies (2010), that differ primarily along midocean ridges and thus at hydrothermal vent sites. However, model studies have shown that regardless of the magnitude of the geothermal heat flux added to the system, from 50 mW m 22 (e.g., Adcroft et al 2001) to 100 mW m 22 (Urakawa and Hasumi 2009), whether it is applied as spatially varying or uniform (e.g., EmileGeay and Madec 2009;Mashayek et al 2013) or irrespective of the model configuration used (idealized to ocean-ice general circulation model), it is evident that ''geothermal heating is an important factor of abyssal dynamics, and should no longer be neglected in oceanographic studies '' (Emile-Geay and Madec 2009, p. 203). …”

Bottom Boundary Potential Vorticity Injection from an Oscillating Flow: A PV Pump

“…Surface processes are likely to be intermittent in both space and time, whereas boundary currents may extend over a broad geographic extent along continental slopes and midocean ridges. While this model is too simple to provide a global assessment of its contribution to deep-ocean PV fields, it focuses the attention of future work on boundary processes, consistent with recent assessments of the importance of sloping bottoms on the abyssal overturning circulation (Mashayek et al 2015). .…”